1. Field of the Invention
The present invention relates to optical transmission devices, and more particularly, to an optical transmission device for transmitting a wavelength-multiplexed signal beam.
2. Description of the Related Art
It is sometimes the case that when an optical transmission system is constructed in a certain area, there is no demand for communications at first and that as a result of subsequent development of the area, the demand for communications rapidly increases. To meet such a varying demand for communications, there has been proposed a method in which an optical transmission device with add/drop function is installed in an area so that the add/drop function may be enabled when the demand for communications rises (see, e.g., Unexamined Japanese Patent Publication No. 2004-297235).
Also, an optical transmission device is known which has no optical wavelength add/drop function (add/drop unit) at first, in order to reduce costs, and an add/drop unit is added as needed after the service is started. When the configuration of the optical transmission device is changed by adding the add/drop unit thereto, it is necessary that the time of signal interruption should be minimized (to about 50 ms). Accordingly, an optical switch or the like is arranged in the route from the add/drop unit to be added so that the add/drop function can be activated in a moment.
FIG. 12 is a block diagram of an optical transmission device to which an add/drop unit is additionally attached. As illustrated, the optical transmission device has a coupler 101, an add/drop unit 102, and an optical switch 103.
The coupler 101 splits a WDM (Wavelength Division Multiplexing) signal beam received from an optical network and outputs the split signal beams to optical paths A101 and A102, respectively. The optical switch 103 outputs the signal beam, received from either the optical path A101 or A102, to a subsequent stage. The add/drop unit 102 is capable of adding/dropping a signal beam and is removably inserted in the optical path A101.
Where it is unnecessary to add/drop a signal beam, the add/drop unit 102 is detached from the optical transmission device. In this case, the optical switch 103 is controlled such that the signal beam from the optical path A102 is output to the subsequent stage.
If the need to add/drop a signal beam arises after the provision of service by the optical transmission device is started, the add/drop unit 102 is inserted in the optical path A101, as illustrated in FIG. 12. In this case, the optical switch 103 is controlled such that the signal beam from the add/drop unit 102 is output to the subsequent stage.
In this manner, after the add/drop unit 102 is attached to the optical transmission device, the optical switch 103 switches the path from the optical path A102 to the optical path A101, whereupon the configuration of the optical transmission device is instantly switched from ILA (In-Line Amplifier) configuration to OADM (Optical Add/Drop Multiplexer) configuration, thus shortening the signal interruption time. The ILA configuration denotes the configuration of the optical transmission device without the add/drop unit 102, and the OADM configuration denotes the configuration equipped with the add/drop unit 102.
When the optical transmission device is switched from the ILA to the OADM configuration, however, the transmission quality of the signal beam lowers, depending on the state of this optical transmission device or the states of other optical transmission devices which constitute the same network and of which the configuration is not switched.
In FIG. 12, for example, if the configuration of the optical transmission device is switched even though the add/drop unit 102 is anomalous, signal interruption occurs, lowering the transmission quality. Also, if the configuration of the optical transmission device is switched while signal degradation is caused in the add/drop unit of a different optical transmission device whose configuration is not switched, the transmission quality lowers.